Chapter 9 TRAP Routines and Subroutines

1. Chapter 9TRAP Routines andSubroutines

2. Subroutines • A subroutine is a program fragment that: • lives in user space • performs a well-defined task • is invoked (called) by another user program • returns control to the calling program when finished • Like a service routine, but not part of the OS • not concerned with protecting hardware resources • no special privilege required • Reasons for subroutines: • reuse useful (and debugged!) code without having tokeep typing it in • divide task among multiple programmers • use vendor-supplied library of useful routines

3. JSR Instruction • Jumps to a location (like a branch but unconditional),and saves current PC (addr of next instruction) in R7. • saving the return address is called “linking” • target address is PC-relative (PC + Sext(IR[10:0])) • bit 11 specifies addressing mode • if =1, PC-relative: target address = PC + Sext(IR[10:0]) • if =0, register: target address = contents of register IR[8:6](called "JSRR" in assembly language)

4. JSR NOTE: PC has already been incrementedduring instruction fetch stage.

5. JSRR Instruction • Just like JSR, except Register addressing mode. • target address is Base Register • bit 11 specifies addressing mode • What important feature does JSRR providethat JSR does not?

6. JSRR NOTE: PC has already been incrementedduring instruction fetch stage.

7. RET (JMP R7) • How do we transfer control back toinstruction following the TRAP? • We saved old PC in R7. • JMP R7 gets us back to the user program at the right spot. • LC-3 assembly language lets us use RET (return)in place of “JMP R7”. • Must make sure that service routine does not change R7, or we won’t know where to return.

8. Saving and Restoring Registers • Called routine -- “callee-save” • Before start, save any registers that will be altered(unless altered value is desired by calling program!) • Before return, restore those same registers • Calling routine -- “caller-save” • Save registers destroyed by own instructions orby called routines (if known), if values needed later • save R7 before TRAP • save R0 before TRAP x23 (input character) • Or avoid using those registers altogether • Values are saved by storing them in memory.

9. Saving and Restoring Registers • Must save the value of a register if: • Its value will be destroyed by service routine, and • We will need to use the value after that action. • Who saves? • caller of service routine? • knows what it needs later, but may not know what gets altered by called routine • called service routine? • knows what it alters, but does not know what will be needed later by calling routine

10. Example: Negate the value in R0 • 2sComp NOT R0, R0 ; flip bits ADD R0, R0, #1 ; add one RET ; return to caller • To call from a program (within 1024 instructions): • ; need to compute R4 = R1 - R3 ADD R0, R3, #0 ; copy R3 to R0JSR 2sComp ; negate ADD R4, R1, R0 ; add to R1 ... • Note: Caller should save R0 if we’ll need it later!

11. Passing Information to/from Subroutines • Arguments • A value passed in to a subroutine is called an argument. • This is a value needed by the subroutine to do its job. • Examples: • In 2sComp routine, R0 is the number to be negated • In OUT service routine, R0 is the character to be printed. • In PUTS routine, R0 is address of string to be printed. • Return Values • A value passed out of a subroutine is called a return value. • This is the value that you called the subroutine to compute. • Examples: • In 2sComp routine, negated value is returned in R0. • In GETC service routine, character read from the keyboardis returned in R0.

12. Using Subroutines • In order to use a subroutine, a programmer must know: • its address (or at least a label that will be bound to its address) • its function (what does it do?) • NOTE: The programmer does not need to knowhow the subroutine works, butwhat changes are visible in the machine’s stateafter the routine has run. • its arguments (where to pass data in, if any) • its return values (where to get computed data, if any)

13. Saving and Restore Registers • Since subroutines are just like service routines,we also need to save and restore registers, if needed. • Generally use “callee-save” strategy,except for return values. • Save anything that the subroutine will alter internallythat shouldn’t be visible when the subroutine returns. • It’s good practice to restore incoming arguments to their original values (unless overwritten by return value). • Remember: You MUST save R7 if you call any othersubroutine or service routine (TRAP). • Otherwise, you won’t be able to return to caller.

14. Example • Write a subroutine FirstChar to: find the first occurrenceof a particular character (in R0) in a string (pointed to by R1); return pointer to character or to end of string (NULL) in R2. • (2) Use FirstChar to write CountChar, which: counts the number of occurrences of a particular character (in R0) in a string (pointed to by R1);return count in R2. • Can write the second subroutine first, without knowing the implementation of FirstChar!

15. save regs count <- 0 count <- count + 1 R1 <- R2 + 1 call FirstChar save R7,since we’re using JSR Look at M[R2] R2 <- count restore regs =0? no return yes CountChar Algorithm (using FirstChar)

16. CountChar Implementation • ; CountChar: subroutine to count occurrences of a charCountChar ST R3, CCR3 ; save registers ST R4, CCR4ST R7, CCR7 ; JSR alters R7ST R1, CCR1 ; save original string ptr AND R4, R4, #0 ; initialize count to zeroCC1 JSR FirstChar ; find next occurrence (ptr in R2) LDR R3, R2, #0 ; see if char or null BRz CC2 ; if null, no more chars ADD R4, R4, #1 ; increment count ADD R1, R2, #1 ; point to next char in string BRnzp CC1CC2 ADD R2, R4, #0; move return val (count) to R2 LD R3, CCR3 ; restore regs LD R4, CCR4 LD R1, CCR1 LD R7, CCR7 RET ; and return

17. R3=R0? save regs yes R2 <- R1 no R2 <- R2 + 1 R3 <- M[R2] restore regs =0? no return yes FirstChar Algorithm

18. FirstChar Implementation • ; FirstChar: subroutine to find first occurrence of a charFirstChar ST R3, FCR3 ; save registers ST R4, FCR4 ; save original char NOT R4, R0 ; negate R0 for comparisons ADD R4, R4, #1 ADD R2, R1, #0 ; initialize ptr to beginning of stringFC1 LDR R3, R2, #0 ; read character BRz FC2 ; if null, we’re done ADD R3, R3, R4 ; see if matches input char BRz FC2 ; if yes, we’re done ADD R2, R2, #1 ; increment pointer BRnzp FC1FC2 LD R3, FCR3 ; restore registers LD R4, FCR4 ; RET ; and return

19. Library Routines • Vendor may provide object files containinguseful subroutines • don’t want to provide source code -- intellectual property • assembler/linker must support EXTERNAL symbols(or starting address of routine must be supplied to user) • ... .EXTERNAL SQRT • ... LD R2, SQAddr ; load SQRT addr JSRR R2 ...SQAddr .FILL SQRT • Using JSRR, because we don’t know whether SQRTis within 1024 instructions.